1. Field of the Invention
The present invention relates to a display, active matrix substrate, and driving method and, more particularly, to a display which controls the optical characteristic of a display element by a current to be supplied to it, an active matrix substrate usable for the display, and a driving method of the display.
2. Description of the Related Art
In a display such as an organic EL (ElectroLuminescence) display which controls the optical characteristics of a display element by a drive current to be supplied to it, if the drive current varies, the image quality becomes poor due to, e.g., uneven luminance. When such a display employs an active matrix driving system, the drive transistors which drive the display elements must have almost uniform characteristics between the pixels. In this display, however, since the transistors are normally formed on an insulating body such as a glass substrate, the transistor characteristics readily vary.
To solve this problem, U.S. Pat. Nos. 6,229,506 and 6,373,454B1 propose circuits shown in FIGS. 1 and 2, respectively. Characteristic correction of drive transistors by using these circuits will be described below.
In an organic EL display 1 using the circuit (threshold value cancel type) shown in FIG. 1, to set a given pixel 2 in a display state, an output control switch Sw2 is opened (OFF) by using a scan signal line 7 first. Simultaneously, a correction switch Sw3 is closed (ON) by using a scan signal line 15 to supply charges to capacitors C1 and C2 until no current flows between the source and drain of a drive transistor Tr. In this state, since the drain and gate of the drive transistor Tr are connected, the potential at a point A is equal to a threshold value Vth of the drive transistor Tr. During this time, a scan signal is supplied from a scan signal line driver (not shown) to a scan signal line 6 to close a selection switch Sw1. At the same time, a reset signal Vrst is supplied from a video signal line driver (not shown) to a video signal line 9.
After the above operation is completed, the correction switch Sw3 is opened, and the output control switch Sw2 is closed. In addition, a video signal Vsig is supplied from the video signal line driver to the video signal line 9. Accordingly, the gate potential of the drive transistor Tr varies from the threshold value Vth by an amount equal to the variate from Vrst to Vsig. As a result, a drive current corresponding to the variation amount is supplied from a power supply line 11 to an organic EL element 20 through the drive transistor Tr and output control switch Sw2.
As described above, according to the circuit shown in FIG. 1, the influence of the threshold value Vth on the drive current can be eliminated. Hence, even when the threshold value of the drive transistor Tr varies between the pixels 2, the influence of such variation on the drive current to be supplied to the organic EL element 20 can be minimized.
However, the drive current is affected not only by the threshold value of the drive transistor Tr but also by its mobility and dimensions. For this reason, it is difficult in the circuit shown in FIG. 1 to so improve the light emission uniformity that no display nonuniformity is visually recognized.
On the other hand, in the organic EL display 1 using the circuit (current copy type) shown in FIG. 2, to set the given pixel 2 in a display state, the output control switch Sw2 is opened first. Simultaneously, the selection switch Sw1 and a correction/write switch Sw4 are closed. In this state, a current Isig corresponding to the video signal is supplied between the source and drain of the drive transistor Tr by using a constant current circuit (not shown). With this operation, the voltage between the two electrodes of the capacitor C2 becomes the gate-to-source voltage necessary for supplying the current Isig to the channel of the drive transistor Tr.
After that, the selection switch Sw1 and correction/write switch Sw4 are opened, and the output control switch Sw2 is closed. The potential at a point B is set by the above operation to supply a drive current almost equal to the current Isig between the source and drain of the drive transistor Tr.
As described above, according to the circuit shown in FIG. 2, a current having a magnitude almost equal to that of the current Isig supplied as a video signal during the write period can be supplied between the source and drain of the drive transistor Tr even during the holding period next to the write period. For this reason, not only the influence of the threshold value Vth of the drive transistor Tr but also the influence of its mobility and dimensions on the drive current can be eliminated.
In the circuit shown in FIG. 2, however, when the pixel size becomes large, the wiring capacitance of video signal lines increases. In addition, when the write period is shortened along with micropatterning of the structure, the following problem rises. That is, when the current Isig is small, the write becomes insufficient. In other words, it becomes difficult to write a desired video signal.